The analysis found that in 2012, only about 27 percent of American Catholics called themselves "strong Catholics;" that's a down more 15 points since the mid-1980s and among the lowest level since the GSS started asking the question 38 years ago.

"The decline among U.S. Catholics is even starker when they are compared with Protestants, whose strength of religious identification has been rising in recent years," the report finds. "About half (54%) of American Protestants – double the Catholic share (27%) – described their particular religious identity as strong last year, among the highest levels since the GSS began asking the question in 1974."

-- Strength of religious affiliation is connected to the frequency of attendance to worship services. So, as you may have guessed, "the share of all Catholics who say they attend Mass at least once a week has dropped from 47% in 1974 to 24% in 2012."

-- Among protestants "self-reported church attendance has been fairly stable."

-- While the percentage of "strong" Catholics is in decline, "the share of the public that identifies as Catholic but not "strong" Catholic, on the other hand, has risen slightly, from 14% in 1974 to nearly 18% in 2012."